Exposure control for photographic cameras



Aug. 1, 1961 M. w. LA RUE, JR., EI'AL 2,994,256

EXPOSURE CONTROL FOR PHOTOGRAPHIC CAMERAS Original Filed Dec. 17, 1956 6Sheets-Sheet 1 m xl N Aug. 1, 1961 M. W.-=LA RUE, JR., EI'AL EXPOSURECONTROL FOR PHOTOGRAPHIC CAMERAS Original Filed Dec. 17, 1956 6Sheets-Sheet 2 Aug. 1, 1961 M. w. LA RUE, JR., ETAL EXPOSURE CONTROL FORPHOTOGRAPHIC CAMERAS 6 Sheets-Sheet 3 Original Filed Dec. 17, 1956 152/672 to 7'5. Merl/z n Z'l/f [aRugdi ZWZ'Z zman lz'am 701 10gb w m. a

Aug. 1, 1961 M. w. LA RUE, JR., ETAL 2,994,256

EXPOSURE CONTROL F OR PHOTOGRAPHIC CAMERAS 6 Sheets-Sheet 4 OriginalFiled Dec. 17, 1956 Merl/2" I'IAI/I/A William I J IIIIIIII/ IIIIMIHlllllllllllll'llllllllllu Aug. 1, 1961 M. w. LA RUE, JR., ETAL2,994,256

EXPOSURE CONTROL FOR PHOTOGRAPHIC CAMERAS Original Filed Dec. 17, 1956 6Sheets-Sheet 5 Aug. 1, 1961 M. w. LA RUE, JR, ETAL 2,994,256

EXPOSURE CONTROL FOR PHOTOGRAPHIC CAMERAS 6 Sheets-Sheet 6 OriginalFiled Dec. 17, 1956 4 1722572102" Merl/2'72 Zfl ZaZPuQ, J7:Zflz'llz'am 1. Mi 0237 M? United States Patent 2,994,256 EXPOSURECONTROL FOR PHOTOGRAPHIC CAMERAS Mervin W. La Rue, Jr., Park Ridge, andWilliam W. Wightman, Niles, Ill., assigiors to Bell & Howell Company,Chicago, 111., a corporation of Illinois Original application Dec. 17,1956, Ser. No. 628,753. Divided and this application June 18, 1958, Ser.No.

12 Claims. (01. 95 10 This invention relates to apparatus for settingthe iris of a photographic camera to insure proper exposure of the filmtherein under the conditions of the light picked up by the apparatus,and has for its general object the provision of a new and improvedapparatus of this kind, this application being a division of ourcopending application, Serial Number 628,753, filed December 17, 1956,for exposure control for photographic cameras.

The specific object of the invention is to provide in a camera exposurecontrol embodying a photoelectric cell and a galvanometer in circuittherewith, temperature compensating means whereby the exposure controlfunctions uniformly over a wide range of temperatures.

Further objects of the invention not specifically mentioned here will beapparent from the detailed description and claims which follow,reference being had to the accompanying drawings in which a preferredembodiment of the invention is shown by way of example only and inwhich:

FIG. 1 is a front elevational view of a portable motion picture cameraequipped with the devices of the present invention;

FIG. 2 is a side elevational view of the camera shown in FIG. 1;

FIG. 3 is a fragmentary cross sectional view taken along the line 33 ofFIG. 1, looking in the direction of the arrows;

FIG. 4 is an elevational View of the rear of the front plate of thecamera as seen from the line 44 of FIG. 3, looking in the direction ofthe arrows with certain parts shown in section;

FIG. 5 is a fragmentary cross sectional view taken along the line 55 ofFIG. 3, looking in the direction of the arrows;

FIG. 6 is a fragmentary cross sectional view taken substantially alongthe line 66 of FIG. 3, looking in the direction of the arrows;

FIG. 7 is a plan view of the motor compartment of the camera as seenalong the line 7-7 of FIG. 2, looking in the direction of the arrows andhaving the motor compartment cover removed;

FIG. 8 is a fragmentary cross sectional view along the line 8-8 of FIG.3, looking in the direction of the arrows;

FIG. 9 is a cross sectional View along the line 9-9 of FIG. 8, lookingin the direction of the arrows and drawn to an enlarged scale;

FIG. 10 is a bottom view, partly in section, along the line -1010 ofFIG. 3;

FIG. 11 is a schematic circuit diagram of the invention; and

FIG. 12 is a graph showing the temperature resistance characteristics ofthe galvanometer circuit.

In order to properly expose the photographic film with which a camera isloaded, it is preferable that a light meter be used to measure theconditions of light under which the picture is to be taken, thereby todetermine the proper setting of the iris of the camera so as to admitthe proper amount of light to the film at the shutter speed at which thecamera is set. The present invention obviates the necessity of using aseparate light meter by incorporating in the camera itself aphotoelectric cell by which the light value is determined, together withapparatus for automatically setting the iris of the camera in accordancewith this light value and the shutter speed at which the camera is set.In the embodiment of the invention shown by way of example, the settingof the his is fully automatic, being accomplished by an electric motorthat is connected by a gear train to the iris ring of the camera. Anelectric switch is conveniently actuated by the button through which thecamera is controlled thereby to effect setting of the iris under theexisting light conditions just previous to exposure of the fihn.

The invention will be best understood by reference to the accompanyingdrawings in which there is shown a portable motion picture camera of themagazine loading type to which the devices of the invention are applied.

The camera comprises the usual front plate 10 in which the photographiclens barrel 11 and front portion of the finder 12 are mounted in theusual manner. As shown, the lens barrel incorporates a focusing type oflens mounting, this particular mounting being shown by way of example asthe invention may equally well be used with -a fixed focus lens ifdesired.

Also mounted upon the front plate 10 of the camera in downwardly spacedparallelism with the photographic lens, is a tubular member 13, andlocated at the bottom of the camera is a motor compartment 14 in whichthe driving motor and batteries therefor are mounted, as will presentlyappear. The remainder of the camera, that is the main body thereof andinstrumentalities therein contained, are standard and need not here bedescribed in detail.

As will best be seen in FIGS. 3 and 10, the tubular member 13 is mountedfor angular adjustment in a forwardly extending boss 15 on the lowerportion of the front plate 10 of the camera, and the tubular member isso mounted by a mounting ring 17 provided with an intermediatelydisposed internal flange 18 rearwardly engaged on an external flange 19on the tubular member 13 and bearing rearwardly against the front of theboss 15, and the mounting ring having its rear portion engaged on theexterior of the boss 15 and secured thereon by set screws 20.

Mounted in the forward portion of the tubular member 13 is aphotoelectric cell 21, together with a light controlling bafile 22 andreticular lens 23, all of which are held in by a mounting ring 23 whichsecures these devices in the barrel. The photoelectric cell isaccessible to light from forwardly of the camera, and preferably thebaffle 22 and the lens 23 of the photoelectric cell unit are designed toaccept light from a field equal to or closely approximating the field ofthe photographic lens that is mounted in the lens barrel 11. Throughthis arrangement the light value by which the setting of the iris iscontrolled is substantially the same as the light that will be admittedto the camera to expose the film when the shutter is open.

Mounted in the tubular member 13 in concentric relation therewith and inthe rear portion thereof rearwardly of the photoelectric cell, is agalvanometer 24 which is secured in the tubular member by set screws 25.(FIGS. 3, 6 and 10). The galvanometer, which is of a usual type,comprises a permanent magnet stator 26 and a coil 27 arranged in thecenter thereof and rotatable coaxially thereof and consequentlycoaxially of the tubular member 13 by means of a rotatably mounted shaft28 upon which the coil is mounted. The coil is biased as usual into azero position by usual spring means not shown, and fixed with and thusrotatable with the coil, is a rearwardly projecting contact 29.

An annular stop member 30 is mounted on the front plate 10 of the camerawithin the bore of the boss 15 and rearwardly of the galvanometer 24(FIGS. 3 and 5), and forwardly projecting spring fingers 31 on the stopmember bear forwardly against the inner end of the tubular member 13 tomaintain the tubular member flange 19 in frictional engagement with themounting ring flange 18 and thus maintain the tubular member in positionangularly on the boss 15 while permitting angular adjustment of thetubular member for a purpose hereinafter appearing.

Projecting forwardly from the stop member 30 are spaced apart lugs 34which lie in the path of the rear bearing bracket 35 by which the rearend of the shaft 28 of the galvanometer is supported. Engagement of thisrear mounting bracket 35 with the stops limits the rotation of thetubular member 13 and apparatus carried thereby around the axis of thebarrel.

As will be seen best in FIG. 5, stop member 30 carries an inwardlyextending portion 36 that terminates in spaced apart stops 37 which liein the path of the movable contact 29 that is carried with the coil 27of the galvanometer, these stops serving to limit the movement of thecoil 27 around the axis of the shaft 28 upon which the coil is mounted.

Stop member 30 is mounted within the boss 15 by screws 38 screwthreadedinto the front plate and projecting through angularly extending slots 39in the stop member 30, thereby to permit limited adjustment of the stopmember to position stops 34 and 37 in a desired position angularly ofthe axis of the tubular member 13. This adjustment is set during initialassembly and calibration of the apparatus, and usually will remain inthis set position throughout the life of the apparatus.

As will be seen best in FIG. 2, the tubular member 13 carries anindexing scale 32 and the mounting ring 17 carries a cooperatingindexing scale 33. The markings of the scale 32 on the tubular membermay be designated and positioned to correspond to the emulsion speedratings of various films according to the A.S.A. system, and themarkings of the scale 33 on the mounting ring may be designated andpositioned according to the shutter speeds of the camera, which in thecase of the motion picture camera shown may vary from sixteen frames persecond to sixty-four frames per second. By rotating the tubular member13 and instrumentalities carried thereby around the axis of the barrel,the mark of the scale 32 corresponding to the emulsion speed of the filmwith which the camera is loaded, is registered with the mark of thescale 33 corresponding to the shutter speed for which the camera is set,thereby to adjust the apparatus to properly set the iris of the camerafor that particular film at that shutter speed as hereinafter described.

Fixed upon the inner face of the front plate 10 of the camera (FIGS. 3and 4), is a mounting base or plate 40 that is positioned inwardly ofthe camera by bosses 41 to which the base is secured in convenientmanner as by screws 42.

Fixed in the mounting base 40 and extending therethrough, near thebottom thereof, is an axially bored arbor 43 upon which is journaled agear 44. The gear 44 is of insulating material and fixed on its frontface in angularly spaced relation are conductive plates 45 upon whichtwo forwardly disposed contacts 46 are respectively mounted in spacedrelation angularly of the gear, these plates and contacts beinginsulated from each other and from the remainder of the structure byreason of the insulating character of the gear 44. The movable contact29 carried by the coil 27 of the galvanometer extends rearwardly fromthe galvanometer coil and between the spaced apart contacts 46, as willbe seen best in FIGS. 4 and 10, for alternate contact therewith withangular movement of the galvanometer coil in opposite directions, thecontact 29 and contacts 46 constituting a double throw switch.

As shown, the photographic lens barrel 11 carries within it a suitablephotographic lens 47 and a conventional iris which comprises (FIG. 3) aplurality of iris leaves 48 arranged in overlapping relation about thelens axis and individually pivotally mounted for opening and closingmovement by means of pivot pins 49 pivotally mounted on the lens barrelin angularly spaced relation about the lens axis, control pins 50secured on the leaves in uniformly radially spaced relation with thepivot pins and arranged about the lens axis, a control ring 51 rotatablymounted within the lens barrel in coaxial relation with the lens axisand provided with cam slots 52 uniformly angularly spaced thereon andrespectively slidably engaged by the control pins, an iris adjustingring 53 rotatably mounted externally on the lens barrel 11, and aconnecting pin 54 secured on the adjusting ring and extending radiallyinward thereof through an angular slot 55 through the lens barrel andengaging in a slot 56 in the control ring 51, so that angular movementof the iris adjusting ring elfects opening or closing movement of theiris depending on the direction of movement of the iris adjusting ring.For the purpose of the invention hereof, the iris adjusting ring 53 isin the form of a gear.

Journaled upon an arbor 57 carried by the mounting base 40 is a clusterconsisting of a center gear 58, at one end of which is fixed a largerdiameter gear 59 that is meshed with the iris adjusting ring gear 53.Included in the cluster with gear 58 is another gear 60 which in FIG. 4is shown meshed with a gear 61 that is journaled upon a pin 62 that iscarried by the mounting base. Gear 61 meshes with a gear 63 that isjournaled upon the arbor 43 and separated from the gear 44 thereon by awasher 64. Gear 63 is meshed with a gear 65 that is fixed upon a shaft66 (FIG. 9) that is journaled in a bushing '67 fixed in the mountingbase 40. Through this arrangement, rotation of the shaft 66 rotatesgears 65, 63, 61, 60, 58 ,59 and 53, to move the iris from one positionto another. Gear 58 meshes with and rotates the gear 44 and spaced apartcontacts 46 carried thereby in correspondence with rotation of the irisring adjusting gear 53 for a purpose which will presently appear.

In order to rotate the shaft 66, an electric motor 70,

which, as shown, is of the permanent field magnet type, is

provided in the motor compartment 14 where it is mounted in suitablebrackets and secured in the compartment preferably by screws, as shownin FIGS. 7 and 8. The shaft 71 of the motor carries a worm 72 thatmeshes with a gear 73 on a cross shaft 74 that is rotatably mounted inthe bracket 75 fixed in the motor compartment. A worm 76 on the crossshaft 74 meshes with a gear 77 rotatably mounted on a shaft 78 which isrotatably mounted on the bracket 75 and is alined with the shaft 66(FIG. 9). Shaft 78 carries a cup shaped coupling member 79 provided withdiametrically opposite slots 80, and a pin 81 is secured on the shaft 66in diametrically extending relation therewith and slidably engages inthe slots to form a drive coupling between the shafts 78 and 66.

Disposed within an enlargement of the bore of the gear 77 are frictiondisks 82 of yieldable material, which together with the web of this gearand a rigid end washer 82' are compressed together between C springwashers 83 which are engaged respectively in axially spaced grooves onthe shaft 78, thus forming a yieldable friction drive between the gear77 and the shaft 78. The drive between the motor shaft 71 and the shaft66 constitutes a yieldable reduction drive which permits of a relativelysmall low power motor and which permits of rotation of the shaft 66relative to the motor.

Also mounted in the motor compartment are a plurality of battery cells84 connected together in series and to an electrical contact 85 at oneend of the series and to a contact spring 86 at the other end thereof(FIG. 7). A contact spring 87 makes contact with the series of batterycells at the mid-point thereof, the contact 85 and the contact springs86 and 87 being fixed in the compartment and insulated from the camerastructure and from each other by suitable insulators respectivelydesignated at 88 and 89.

Projecting through a slot in the side wall of the front plate of thecamera (FIG. 4) is a camera control button 91 that is slidable in thisslot to initiate and terminate operation of the camera to expose thefilm therein. Located inwardly of this button is a contact spring 92which is mounted on the button 91 and is in electrical contact with theframe of the camera and normally spaced away from a fixed contact 93that is insulated from the frame. A spring 94 encircles the stem of thebutton 91 and bears against the finger spring 92 to maintain it out ofengagement with the fixed contact 93. By pressing on the button 91 tomove it inwardly of the camera, finger spring 92 is moved intoengagement with the fixed contact 93.

The inward movement of the camera control button 91 to engage thecontact spring 92 with the contact 93 is a natural incident of manualengagement of the control button to effect operation of the camera, sothat normally these contacts are engaged during operation of the cameraand are disengaged when the control button is released and operation ofthe camera is thus terminated. Engagement of the contact spring 92 withthe contact 93 causes the exposure control to function, and it isaccordingly desirable to engage these contacts before moving the controlbutton 91 into camera operating position so that the exposure controlhas time to adjust the iris to the instant light condition before anyfilm is exposed.

As will be seen in the schematic circuit (FIG. 11), one terminal of thephotoelectric cell is connected to the frame of the camera at 95, whichconnection is for convenience referred to as ground, and at its otherterminal is connected by conductor 96 that leads to a thermistor 97 thatis shunted by a modifying resistor 98, the circuit continuing through atrimming resistance 99 over conductor 100 through the winding coil 27 ofthe galvanometer to ground at 101.

The current generated by the light falling on the photoelectric cell 21thus passes through the galvanometer coil 27 and tends to move it andthe movable contact 29 around the axis of the coil from theaforementioned spring biased zero position thereof to a position,dependent on the instant intensity of the light impinging on this celland resulting corresponding potential on the coil, in which theresultant magnetic attraction between the coil and galvanometer statorbalances the spring bias on the coil.

Still referring to FIGURE 11, a power circuit for the motor 70 comprisesthe contact spring 92 connected to ground 102, the contact 93 connectedto one end of the motor winding by a conductor 103, a conductor 104connecting the other end of the motor winding to the spring contact 87which contacts the battery cells 84 at the mid-point thereof, aconductor 105 connecting one of the contact plates 45 and with it thecorresponding contact 56 with the contact 85 which contacts one end ofthe battery cells 84, a conductor 106 connecting the other of thecontact plates 45 and with it the corresponding contact 46 with thecontact 86 which contacts the oher end of the battery cells 84, and theconnection of the movable contact 29 to ground 101. The conductors 105and 106 conveniently pass from in front of the gear 44 where they arerespectively secured to the fixed contacts 46, through the bore of thearbor 43 and into the motor compartment 14, as shown in FIGURE 4.

Noting that the movable contact 29 is arranged between the spacedcontacts 46, and forms double throw switch therewith, the movablecontact may engage either spaced contact or may assume an intermediateposition therebetween in which it is disengaged from both spacedcontacts, and assuming the spring contact 92 to be engaged with thecontact 93, engagement of the movable contact with the righthand spacedcontact 46, as seen in FIG. 11, closes a circuit through the motor 70from the group of battery cell 84 connected between the battery contacts85 and 87 and engagement of the movable contact with the letfthandspaced contact 46 closes a circuit through the motor from the group ofbattery cells connected between the battery contacts 86 and 87. Thebattery cells being arranged in series and the intermediate batterycontact 87 being common and the battery contacts 85 and 86 beingindividual to these circuits, potential of opposite polarities issupplied to the motor respectively through these circuits, so that themotor armature and its shaft 71 rotates in one direction or the otherdepending on which spaced contact 46 the movable contact 29 engages.

Recalling that the motor 70 is connected to adjust in correspondence theiris adjusting ring gear 53 and with it the iris and the gear 44 and thespaced contacts 46 carried thereby, the operation of the exposurecontrol will now be described.

Having positioned the camera to photograph the desired scene, lightfalling upon the photoelectric cell 21 causes the movable contact 29 tobe urged to a position corresponding with the intensity of the instantlight as hereinbefore described, and as a result, except where the irishappens to be already adjustable in correspondence with the instantlight, the movable contact 29 is engaged with one or the other of thefixed contacts 46 depending on whether the opening of the iris is to beincreased or decreased to correspond with the instant light. This closesthe corresponding motor circuit and as a result the motor is energizedwith potential of the proper polarity and adjusts the iris and gear 44and the fixed contacts 46 carried thereby to correspond with the instantlight, the movable contact 29 following and maintaining engagement withthe corresponding spaced contact 46 until the movable contact 29 reachesthe position into which it is urged by the instant potential of thephotoelectric cell, whereupon the engaged spaced contact disengages fromthe movable contact, thus deenergizing the motor and terminatingadjustment of the iris and gear 44 with the iris adjusted incorrespondence with the instant light. This action is repeated each timethe flight falling on the photoelectric cell changes.

Recalling that the galvanometer 25 is mounted within the tubular member13 which is rotatably mounted in the boss 15 of the front plate 10(FIGS. 3 and 10), the galvanometer is angularly adjustable about itsaxis, relative to the gear 44 carrying the contacts 46 and to the plate30, by angular adjustment of the tubular member 13, this angularadjustment being limited by the lugs 34 of the plate 30 engaging therear bearing bracket 35 of the galvanometer as best shown in FIG. 6.Such adjustment of the galvanometer results in corresponding adjustmentof the zero position of the contact 29, so that the automaticpositioning of the iris for a given amount of light impinging on thephotoelectric cell is correspondingly adjusted. It is this adjustmentwhich is utilized to adjust the mechanism in correspondence with theemulsion speed of the film being used and with the instant shutterspeed, the scales 32 and 33 cooperating for predeterminately adjustingthe mechanism for these factors, as hereinbefore described.

Photoelectric cells when activated by light passing through the lens andbaflies protecting them, produce very small currents, and as a resultthe force tending to rotate the coil 27 of the galvanometer is small andthe contact pressure between the movable contact 29 and the spacedcontact 46 into engagement with which it is thus driven is likewisesmall. The current drawn by the motor is relatively large when comparedwith the current driving the movable coil 27 of the galvanometer, andsince this motor current passes through the movable and spaced contacts29 and 46 which are maintained together by light pressure, there is atendency for these contacts to be Welded together by the heat generatedby this current. To guard against this welding of the contacts, we haveoffset the axis around which gear 44 rotates with respect to the axis ofthe tubular member 13 on which the coil 27 rotates. As a result of thisconstruction, the movable contact 29 and spaced contacts 46 rotate indeviating paths respectively designated at 107 and 108, with the resultthat there is a rubbing action between the movable contact 29 and thespaced contact 46 engaged thereby as the mechanism is operated. It hasbeen found that this wiping action of the contacts prevents weldingtogether thereof, notwithstanding that the current carried through thecontacts is relatively high and the engaging pressure therebetween isrelatively low.

In order to advise the operator of the camera that the light conditionsencountered are approaching the minimum required for proper exposure ofthe film with which the camera is loaded, at the speed at which theshutter is set, we have provided a gear 110 (FIG. 4) that is meshed withthe iris adjustment ring gear 53 and carries a crank pin v111. Pivotedupon the mounting base 40 is a lever 112 which is yieldably urgeddownwardly by gravity and is provided with an edge 113 of which lies inthe path of pin 111 as the gear 110 is rotated. Lever 112 contains anoffset extension 114 (FIG. 3) which carries an indicator 115 movablewith the lever into and out of indicating position visible through theview finder.

The arrangement is such that with final movement of the iris adjustingring gear 53 and iris into the maximum exposure postion of the iris, thegear 53 moving clockwise and the gear 110* moving counter clockwise inFIG. 4, the crank pin 111 engages upwardly against the edge 113 andmoves the lever 112 to position the indicator 115 in its indicatingposition visible through the view finder, and with initial movement ofthe gear 53 and iris out of the maximum exposure position of the iris,the gear 53 moving counter clockwise and the gear 110 moving clockwise,the crank pin moves downwardly and permits the lever 112 and theindicator 115 to move downwardly to position the indicator out of itssaid indicating position.

In the installation shown in the drawings by way of example, thephotoelectric cell 21 is a commercial cell one and one-half inches (l/z") in diameter and the galvanometer 25 is a commercially availableunit having a 100 coil movement in the range of zero to 50 microamperes.A photoelectric cell and galvanometer circuit per se has a positivetemperature coefficient of resistance, i.e., its resistance increaseswith increase of temperature, with the result that for a given intensityof light falling on the photoelectric cell the galvanometer coil will bemoved through an arc whose magnitude decreases with an increase intemperature. In FIG. 12, curve 120 indicates the rise in resistance ofthe photoelectric cell and galvanometer circuit per se occasioned by anincrease in temperature from zero to 100 F.

In order to compensate for this positive temperature coefficient ofresistance of the photoelectric cell and galvanometer circuit per se,thereby to achieve accurate setting of the iris throughout this range oftemperature, we have included in the photoelectric cell and galvanometercircuit a compensating device comprising a resistor which has a negativetemperature coefficient of resistance as designated at 97 in FIG. 11,such devices being commercially known as Thermistors.

A thermistor, having a negative temperature coeflicient of resistancewhich approximately inversely corresponds characteristic of thisthermistor being shown by curve 121 (FIG. 12).

It will be apparent from a comparison of the resistance curves and 121,that the negative temperature coefficient of resistance of the selectedthermistor is greater than the positive temperature coefficient of thephotoelectric cell and galvanometer circuit per se, and thataccordingly, the selected thermistor cannot of itself properlycompensate for the positive temperature coefiicient of resistance of thephotoelectric cell and galvanometer circuit per se. We have therefore,provided a modifying resistor 98 in shunt with the thermistor.Preferably, the resistor 98 is a carbon resistor, and has a zerotemperature coefficient of resistance and approximately 4,000 ohmsresistance as indicated by the line 122 (FIG. 12).

Observing that the temperature coefiicient of resistance of themodifying resistor 98 lies between the positive temperature coeflicientof resistance of the photoelectric cell and galvanometer circuit per seand the negative temperature coefficient of resistance of the thermistor97, the combination of the parallel resistances 97 and 98 results in anoverall negative temperature coefiicient of resistance thereof whichapproximately inversely corresponds with the positive temperaturecoefiicient of resistance of the photoelectric cell and galvanometercircuit per se, the overall resistance of the parallel resistors 97 and98 being indicated by the line 123 (FIG. 12). Accordingly, the inclusionof the parallel resistances 97 and 98 in series with the photoelectriccell and galvanometer coil results in a compensation of the positivetemperature coefficient of resistance of the photoelectric cell andgalvanometer circuit per se, producing a resultant approximately zerotemperature coeflicient of resistance indicated by the line 124 (FIG.12).

The photoelectric cell 21 requires an appreciable amount of resistancein the circuit connected across its terminals, and since the resultantresistance of the circuit comprising the galvanometer coil 27 and theparallel resistances 97 and 98 is in most cases too low, we haveprovided a trimming resistance 99 in series in the circuit, thisresistance preferably being a carbon resistor having a zero temperaturecoefiicient of resistance and its re sistance being sufficient toincrease the overall resistance of the circuit to that required by thephotoelectric cell, as indicated by the line 125 (FIG. 12).

The value required of the resistance 99 varies rather widely indifferent assemblies because of differences in characteristics ofindividual components, and a typical value of the resistor 99 is 1,500ohms. Through the above resistance arrangement the accuracy of thesetting of the iris is maintained within acceptable limits of erroralsrg lglhout the entire range of temperature of zero to In the camerashown by way of example, the motor 70 is designed to operate on threeand one-half (3 /2) volts and draws a maximum of 100 milliamperes whenoperating. The battery cells 84, shown in the drawings, are mercurycells capable of supplying 100 milliamperes of current for a total oftwenty (20) hours with intermittent operation. Twenty (20) hours isestimated to be sufficient running time for the camera to expose 600film magazines. It is therefore postulated that the shelf life of thebattery cells 84 will be the controlling factor in the useful life ofthese cells, and that with average use of the camera this life willdefinitely be in excess of one 1) year and probably in the neighborhoodof two (2) years.

With 100 milliamperes of current drawn as above, it is imperative thatthe motor be not blocked with its circuit closed. To this end, the stops37 on the plate 30 (FIG. 6), by which the range of movement of themovable contact 29 of the galvanometer is controlled, are preferably setso as to limit the movement of the contact 29 to such position that thecontact 46 engaged therewith will break the motor circuit just beforethe iris moves into a limits position. Thus, for example, if the lens 23through which light is admitted to the photoelectric cell is covered, aswith a hand, so that no light falls upon the cell, contact 29 will tendto move to its Zero position, engaging the adjacent contact 46 to closethe motor circuit, thereby to move the iris to fully open position. Asthe gear 44 revolves, the engaged contact 46 is moved in the samedirection as the movable contact 29 is trying to move, and the motorcircuit will remain closed until the contact 29 strikes thecorresponding stop 37, thereby arresting further movement of thiscontact. Contact 46 will then move away from the contact 29, breakingthe motor circuit, and stops 37 are preferably placed so that this willoccur just before the iris is wide open.

Through the same arrangement, should the lens 23 of the photoelectriccell be pointed at an extremely bright light, the galvanometer coil 27will move the movable contact 29 towards its other limits position,moving that contact against the other one of the contacts 46 to closethe motor circuit, thereby to drive the iris ring 53 in the oppositedirection to move the iris to fully closed position until the other stop37 is engaged by the movable contact 29 to break the motor circuit justbefore the iris reaches this position.

With the photoelectric cell 21, galvanometer 25 as above specified, andthe temperature compensation of the galvanometer circuit as above, thearrangement has been found to be sufficiently sensitive over a range oflight value varying from a minimum of 1.5 candles per square foot to amaximum of 1600 candles per square foot, a range which comparesfavorably with the range of photographic light meters ordinarily used.

If it is desired to operate the iris manually, either in eifort to takepictures beyond the range for which the automatic adjusting device issensitive, or for other reasons, such manual operation can be achievedwithout interference from or disabling the automatic adjustmentapparatus. When such manual operation is in progress, it is preferablethat the switch 126 included in the conductor 103 (FIG. 11) be opened sothat the motor circuit will not be closed even though the camera releasebutton 91 be moved inwardly to engage the contact spring 92 with thecontact 93. While it is possible to move the camera release button 91 tostart and stop the camera without closing spring 92 against contact 93,it is preferable to definitely open the circuit of the motor.

The yieldable friction drive, comprising the gear 77 and friction disks82 (FIG. 9) between the motor 70 and the exposure adjusting mechanism,permits rotation of the iris adjusting ring 53 and with it the contactcarrying gear 44 independently of the motor, and to provide for manualrotation of the iris adjusting ring, a manual iris adjusting ring 127(FIGS. 2 and 3), carried by the photographic lens assembly and fixed forrotation with the iris adjusting ring 55, provides for manual adjustmentof the iris. The adjusting ring 127 is provided with an indexing scale128 (FIG. 2) designating adjustments of the adjusting ring in terms ofstandard iris F stop numbers and an index mark 129 on the lens barrel 11adjacent this adjusting ring cooperates with this indexing scale formanually predeterminately adjusting the iris.

Switch 126 has not been shown other than in FIG. 11, since its locationon the camera is a matter of choice. Preferably the switch 126 islocated in a not too accessible place so that its operation to openposition will occur only when the switch is operated deliberately.

While we have chosen to show our invention by illustrating anddescribing a preferred embodiment of it, we have done so by way ofexample only, as there are many modifications and adaptations which canbe made by one skilled in the art within the teachings of the invention.

Having thus complied with the statutes and shown and described apreferred embodiment of our invention, what we consider new and desireto have protected by Letters Patent is pointed out in the appendedclaims.

What we claim is:

1. In a photographic camera, the combination of adjustable exposuremeans for varying the exposure of the camera, a photoelectric cell, agalvanometer having a movable spring biased coil, a circuit connectingsaid cell and coil and comprising a resistor having a negativetemperature coefiicient of resistance for compensating for the positivetemperature coefiicient of resistance of the cell and coil circuit perse, and means under the control of said coil as deflected by lightimpinging on said cell for adjusting said exposure means incorrespondence with the intensity of said light.

2. In a photographic camera, the combination of claim 1 and furthercomprising a trimming resistor included in said circuit in series withsaid first mentioned resistor for increasing that overall resistance ofsaid circuit to that required by said cell.

3. In a photographic camera, the combination of adjustable exposuremeans for varying the exposure of the camera, a photoelectric cell, agalvanometer having a movable spring biased coil, a circuit connectingsaid cell and coil and comprising a resistor having a negativetemperature coefiicient of resistance in excess of the positivetemperature coefficient of resistance of the cell and coil circuit perse and a modifying resistor connected in shunt with said first mentionedresistor and having a temperature coefiicient of resistance lyingbetween those of said first mentioned resistance and the cell and coilcircuit per se, said resistors cooperating for compensating for thepositive temperature coetficient of resistance of the cell and coilcircuit per se, and means under the control of said coil as deflected bylight impinging on said cell for adjusting said exposure means incorrespondence with the intensity of said light.

4. In a photographic camera, the combination of claim 3 and furthercomprising a trimming resistor included in said circuit in series withsaid first and second mentioned resistors for increasing the overallresistance of said circuit to that required by said cell.

5. In a photographic camera, the combination of adjustable exposuremeans for varying the exposure of the camera, a photoelectric cell, agalvanometer having a movable spring biased coil, a circuit connectingsaid cell and coil and comprising a resistor having a negativetemperature coefiicient of resistance having values of approximately50,000 and 2,000 ohms respectively at 0 F. and F., a modifying resistorconnected in shunt with said first mentioned resistors and having a zerotemperature coefficient of resistance and a trimming resistor connectedin series with said first and second mentioned resistors, said first andsecond mentioned resistors cooperating for compensating for the positivetemperature coeflicient of resistance of the cell and coil circuit perse and said trimming resistor being of a value increasing the overallresistance of said circuit to that required by said cell, and meansunder the control of said coil as deflected by light impinging on saidcell for adjusting said exposure means in correspondence with theintensity of said light.

6. In a photographic camera including variable means for adjusting theexposure of the camera, the combination therewith of a photo-electriccell responsive to light from the field of view of the camera, adjustingmeans for ad justing the variable means, a galvanometer for actuatingthe adjusting means and having a coil energized 'by the photo-electriccell and having a positive temperature coeflicient of resistance, firstand second resistors connected in series with the coil and the cell, anda third resistor having a negative temperature coefficient of resistanceconnected in shunt with one of the first and second resistors and inseries with the other of the first and second resistors and the coil andcell, the relative temperature coefiicients of resistance andresistances of the resistors being such as to compensate for change intemperature of the coil and cell with change in temperature.

7. In a photographic camera including variable means for adjusting theexposure of the camera, the combination therewith of a photo-electriccell whose current output decreases with increase in temperature andresponsive to light from the field of view of the camera, adjustingmeans for adjusting the variable means, galvanometer means for actuatingthe adjusting means and having a coil energized by the photo-electriccell and having a positive temperature coefficient of resistance, and anetwork connected in series with the cell and the coil for compensatingfor variations in temperature of the cell and coil and including inshunt relationship relative to one another a first rcsistor havingsubstantially a zero temperature coefiicient of resistance and a secondresistor having a negative temperature coefiicient of resistance.

8. In a photographic light responsive device including a galvanometerand photovoltaic cell means constituting the sole source of electriccurrent to the galvanometer, the combination therewith of a firstresistor having a substantial temperature coefficient connected incircuit with the galvanometer and cell means to control the currentsupplied to the galvanometer, and a second resistor having asubstantially Zero temperature coefficient connected in parallel withthe first resistor, the first and second resistors serving to cause thecurrent to the galvanometer to be substantially constant for a givenillumination of the cell means regardless of the temperature.

9. The photographic light responsive device of claim 8 wherein the firstresistor has a negative coefficient of temperature, and means connectingthe first resistor and the second resistor in series with the cell meansand the galvanometer.

10. In a camera automatic exposure control including a galvanometerhaving a galvanometer provided with a driving coil and a power sourceincluding a photoelectric cell for supplying current to the coil inaccordance with illumination of the cell, the power source constitutingthe sole source of power to the galvanometer, means connecting thegalvanometer and the power source so that all current to thegalvanometer passes through the cell, the combination therewith of afirst resistor having a substantial temperature coefiicient, a secondresistor having a negligible temperature coefficient, and circuit meansconnecting the first and second resistors in parallel with one anotherand in circuit with the coil and the cell to maintain the flow ofcurrent from the cell to the coil constant for a given illumination ofthe cell regardless of temperature.

11. The camera automatic exposure control of claim 10 wherein thephotoelectric cell is of the photovoltaic type and constitutes the solesource of current to the coil, the first resistor has a substantialnegative coelficient of temperature, and the circuit means connects thefirst and second resistors in series with the cell and the coil,

12. In a photographic light responsive device including a galvanometerand a power source including a photoelectric cell through which cell allcurrent to the galvanometer flows, the combination therewith of a firstresistor having a substantial temperature coefficient, a second resistorhaving a substantially Zero temperature coefficient, and meansconnecting the resistors in parallel with one another and to the celland the galvanometer in such a manner as to cause the current to thegalvanometer to be substantially constant for a given illumination ofthe cell regardless of the temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,388,609 Ericsson Nov. 6, 1945 2,626,361 Martine Jan. 20, 19532,655,086 Walker Oct. 13, 1953 2,739,506 Stimson Mar. 27, 1956 OTHERREFERENCES Bell Laboratories Record, Dec. 1940, pages 106 to 111.Electronics-magazine, page 202, vol. 29, No. 7, July 1956.

